The widespread use of polymer-based products ("plastics") in consumer goods has lead to a corresponding interest in suitable methods for reclaiming or recycling as much of these products as is practical. The interest in such recycling is driven by several considerations. These include: environmental factors, principally occupation of landfills and other disposal issues; preservation of limited resources, because polymers are a product in the chain of petrochemicals that begins with crude oil; and economic interests, because the cost of post-consumer plastic is less related to the cost of raw materials, and instead is more dependent upon the cost of physically collecting and transporting post-consumer plastic objects to an appropriate facility for recycling. Additionally, public policy is expected to continue to emphasize environmental issues, and thus legal requirements for recycling are already in force in some jurisdictions, and are expected in others in the future.
Interest in post-consumer recycling has increasingly focused upon polyester (i.e. polyethylene terephthalate, "PET," and related compounds). More specifically, for several decades polyester found its widest use as a synthetic material for textile products. Because polyester is typically blended and textured when incorporated into yarns and fabrics, the task of recycling textile materials is quite different from the task of recycling molded polyester objects.
In more recent years, however, polyester chemistry and the techniques for molding polyester have developed to the point at which polyester has become widely used as a material for "disposable" beverage bottles. Accordingly, these polyester bottles, once disposed of, present a significant opportunity for economical and efficient recycling, provided appropriate techniques can be developed.
In this regard, the term "recycling" often tends to be used relatively broadly, but actually covers a number of different ways to reuse any given material. Thus, in one sense, if a polyester bottle is used to contain a non-food product liquid rather than being directly disposed of, it has been "recycled", but the term "recycled" is rarely applied to such follow-up use of an empty container. Instead, recycling is typically identified as falling into one of three general categories, which are also recognized by the U.S. Environmental Protection Agency (EPA) and the U.S. Food and Drug Administration (FDA). The first is primary recycling; the use of pre-consumer industrial scrap and salvage. The second is secondary recycling, which refers to the physical reprocessing (e.g., grinding and re-extruding) of the polymer. The third is tertiary recycling in which a polymer is reprocessed to isolate chemical components and reuse them in manufacturing a new product.
Because the present invention deals with post-consumer recycling, primary recycling will not be addressed further herein. As used herein, the term "post-consumer" refers to polyester objects (e.g. food containers or bottles) that have been disposed of after their initial use. Because such containers or bottles are generally treated as household trash, or even re-used for non-food compositions, they may contain various contaminants or contaminant residues when submitted for recycling.
With respect to polyester, tertiary recycling generally falls into one of several chemical processes that reduce polyester (typically polyethylene terephthalate) back into its component monomers (terephthalic acid and ethylene glycol) in some sort of depolymerizing step. Typical depolymerizing steps include methanolysis as for example practiced by Eastman Chemical Company, and Hoechst. Shell practices another process referred to as "glycolysis," and which mixes post-consumer polyester with glycol (and potentially other compounds) to break it back down into its component precursors.
Other recycling techniques provide methods for removing companion materials often used with polyester in consumer products. In this regard, U.S. Pat. No. 5,523,329 to Moore et al. for "Recovery of Polyester From Spent Film," which is commonly assigned with the pending application, describes a method of removing polyvinylidene (PVDC) from a polyester substrate, a combination which is often used, for example, as a substrate for photographic film and their emulsions.
In order to be used to package food products, any polymer whether virgin or recycled must meet the appropriate standards for food contact. As used herein, the term "virgin" refers to polymers made directly from chemical precursors, and because of which their purity is generally considered to be acceptably high. Thus, in seeking to produce a food quality polyester from post-consumer polyester, the recycling process must not only produce a reconstituted polyester with appropriate physical and chemical characteristics, but must also be sufficiently pure to meet food quality standards.
In the United States the FDA has generally taken the position that the levels of recycle related contaminants present in food contact articles must be low enough to ensure that the resulting "dietary exposure" to those substances is less than one-half part per billion (ppb). According to FDA calculations, the residual concentration of possible contaminants in the food packaging material should be no more than about 215 ppb. At these levels, if a contaminant were present in a polyester container made from recycled material, and if the contaminant was assumed to migrate entirely (100%) into the food, the concentration of the contaminant in the daily diet would be on the desired order of less than 0.5 ppb based on a 5 percent PET consumption factor (i.e. the FDA assumes that 5 percent of food is packaged in polyester).
At levels greater than about 215 ppb, migration studies can also be used to demonstrate that the concentration in a daily diet would still fall to 0.5 ppb or less.
As noted above, if a polymer such as polyester is manufactured from pure precursor components such as terephthalic acid and ethylene glycol, it can generally be produced in the desired purity ranges. Similarly, and as would be expected, if post-consumer polyester is reduced by glycolysis or methanolysis to the equivalent of pure starting materials, which are then re-reacted to form monomer and re-polymerized to form polymer, a similar purity level can be achieved. The disadvantage, however, of such processes is that by the very nature of their extensive chemical treatment, they are more expensive and less efficient than the processes for producing polyester from virgin precursors. Because one of the objects of recycling is to promote its economic attractiveness, recycling processes that are so elaborate as to offer economic disincentives tend to be used sparingly, and occasionally only in the face of legal requirements, rather than for their marketplace advantages.